1. Field of the Invention
The invention relates to modulation circuits, and more specifically, to pulse duration modulation circuits.
2. Description of the Prior Art
Pulse duration modulation systems are well known in the prior art. In these systems, an intelligence-bearing signal to be impressed on a carrier wave is converted into a variable duration pulse carrier which, in turn, is used to modulate a high frequency sinusoidal carrier wave.
U.S. Pat. No. 2,445,568, issued to J. C. Ferguson on July 20, 1948 concerns such a pulse duration modulation system. As disclosed by Ferguson, a pulse duration modulated pulse carrier is used to switch a vacuum tube having its anode connected directly to the output tank circuit. Thus, the voltage on the anode varies sinusoidally with the amplitude modulated output signal, even though the "subcarrier" current passes through the tube in substantially rectangular pulses.
Since the power loss in the switching tube is a function of the anode voltage, the tube loss is increased during positive excursions of the anode.
Furthermore, prior art devices such as Ferguson's provide no means for overcoming the effects of interelectrode capacitance in the switching tube so that the trailing edges of the subcarrier current pulses tend to decay slowly.
By overcoming the effects of these two characteristics of the prior art devices, circuits constructed in accordance with the principles of the present invention operate at a higher efficiency and lower operating costs.